Electric lamp with light-diffusing coating

ABSTRACT

A glass electric lamp bulb having on the inside surface thereof a light-diffusing coating of self-adherent silica particles having an average particle size, as measured by a scanning electron microscope, within the range from about 0.5 to 1.2 microns.

United States Patent Arsena 1 Sept. 30, 1975 1 1 ELECTRIC LAMP WITH LIGHT-DIFFUSING COATING [56] References Cited [75] Inventor: Vito J. Arsena, Highland Heights, ED TATE PATENTS Ohio 2,806,968 9/1957 'Iliorington et al 313/1 16 X 2,810,660 10/1957 Car enter 313/116 X [73] Asslgncc: General Elecmc 2,877,139 3/1959 Hyd 313/116 x Schenectady, NY.

[22] Filed: Dec. 20, 1973 Primary E.\aminerAlfred L. Brody Appl. NO.Z 426,494

Related [1.8. Application, Data Continuation-impart of Ser. No. 348,347, April 5,

1973, abandoned,

US. Cl. 313/116; 106/52; 313/112; 117/97 Int. Cl. HOlK l/26 Field of Search 117/97;3l3/ll6, 112,110; 220/21 R; 106/54, 52

Arta'rzey, Agent, or FirmEmil F. Sos, Jr.; Lawrence R. Kempton; Frank L. Neuhauser 5 7 ABSTRACT A glass electric lamp bulb having on the inside surface thereof a light-diffusing coating of self-adherent silica particles having an average particle size, as measured by a scanning electron microscope, within the range from about 0.5 to 1.2 microns.

13 Claims, 9 Drawing Figures U.S. Patent Sept. 30,1975 Sheet 1 of6 3,909,649 9 Fig.

H HH I I HH I I HHH H IMH I IH I I I H HHH H H H HUH l l l HH HHHHM ////fl/ ///y/ 9 g g M US. Patent Sept. 30,1975 Sheet20f6 3,909,649

Fig. 4

Fig. 5

US. Patent Sept. 30,1975 Sheet 3 0156 3,909,649

US. Patent Sept. 30,1975 Sheet 4 of6 Photocell Reading vs. Horizontal Scan Along Bulb's Major Diameter.

A= Red Sensitive Celi' B= Blue Sensitive Cell Q-Goated lOOWatt, l2OVolt Lim Fly. 7

Attenuated Beam Reading, 93

0 Horizontal Distance From Edge to Edge of Bulb, D

US. Patent Sept. 30,1975 SheetS 0f6 3,909,649

Photocell Reading vs. Horizontal D. m 0 L w o 8 m w m 2 D l 0 6 W m mmm t wa a.nnm A B nee m 6 "WWW A m= SABS V 8 O m o Horizontal Distance From Edge to Edge of Bulb U.S. Patfint Sept. 30,1975 Sheet 6 of6 SAMPLE= Silica Slurry, Mill 24 Hrs.

WEI ESHT PERCENT GREATER THAN DIAMETER 000 O IOIO (\l DIAMETER IN MICRONS ELECTRIC LAMP WITH LIGHT-DIFFUSING COATING CROSS-REFERENCES TO RELATED APPLICATIONS This application is a continuation-in-part of application Ser. No. 348,347, filed Apr. 5, 1973, now abancloned.

Ser. No. 426,493 Vito J. Arsena, Method of Manufacturing Light-Diffusing Articles, filed concurrently herewith and assigned the same as this invention.

BACKGROUND OF THE INVENTION 1. Field of the Invention The invention relates generally to light-diffusing articles such as incandescent lamps and discharge devices. More particularly, the invention relates to silica-coated bulbs used in the manufacture of electric lamps.

2. Description of the Prior Art Light-diffusing coatings located on the inside surface of a lamp bulb have been desirable in the lighting indus-.

try for some time. One of the first such effective coatings, known as inside frost, was developed by Pipkin, U.S. Pat. 1,687,510, Oct. 16, 1928, and assigned to the assignee of the present invention. The inside frost diffusion coating is manufactured by etching the inside surface of a bulb with hydrofluoric acid. Although Pipkins invention was a great contribution to the lamp art, etching created an environmental problem of acid disposal.

Another diffusion coating widely used in the lamp industry and known as Q-coat was also invented by Pipkin, U.S. Pat. No. 2,545,896. Q-coat is a coating of silica particles predominantly smaller than one micron deposited on the acid etched inside surface of the bulb. Although the silica particles are generally deposited on etched or inside frosted lamps, the Q-coat of silica may be used on smooth or unetched bulbs Q-coat is formed by particle deposition from combustion of an organosilicon compound, such as ethyl orthosilicate. Generally, organosilicon compounds are relatively expensive com pared to naturally occurring silica. It is believed that Pipkins particle range, as recited in the patent, of 0.2 to 0.6 microns is a measurement of numereous agglomerates and that in fact many of the individual particles are smaller than the given range. This apparent discrepancy may be the result of measurements made in the 1940s with less sophisticated measuring techniques that are available at the present time.

Materials other than silica, such as titanium dioxide, have been used as diffusion coatings, and methods, such as electrostatic precipitation, Meister et al., U.S. Pat. No. 2,922,065, are known. In Patent U.S. Pat. No. 3,l75,l l7 Kardos, titanium dioxide particles are interspersed in anorganic binder which is dissolved in an organic solvent to form a slurry which is applied to the inside of the bulb. This slurry is then dried and burned on the inside surface. According to the patent, one type of TiO coating consists of two sizes of particles, one having a diameter below one micron and the other having a diameter between 2 and 4 microns. Inasmuch as the particles are TiO and are tightly packed to one another, presumably because they are dissolved in an organic solvent, the light absorption of this coating is believed to be greater than what is commercially acceptable, namely, 6% or less.

Other slurry mixtures have tried, on etched as well as unetched lamp bulbs, in an attempt to reproduce the frosted and Q-coated bulbs. U.S. Pat. No. 2,661,438 Shand claims the use of colloidal silica particles having a diameter of 0.04 to 0.8 microns mixed with solid silica particles having a diameter between 1 and 15 microns. The mixture is applied to a preheated bulb in an effort to drive off the water of the solution. This coating and method have the disadvantages of poor coating uniformity and short lamp life caused by the coating containing water. Colloidal silica is hydrophilic and absorbs water at room temperature while the bulbs are waiting to be made into lamps.

Various other slurry coatings containing silica of a 2-micron diameter in an organic solution or 2-micron diameter silica mixed with 2-micron diameter titanium dioxide also in an organic solution have been made. However, either light transmittance or filament hiding power is decreased to a point where the lamp is commercially unacceptable. It is believed that these disadvantages are caused by the particles being tightly packed to one another which may be the result of using an organic solvent in the slurry.

SUMMARY OF THE INVENTION It is therefore an object of the invention to provide a lamp bulb with a light-diffusing coating which has relatively low light absorption, that is, less than approximately 6%. Another object of the invention is to provide a lamp bulb with an effective diffusion coating without etching the clear glass bulb through the use of hydrofluoric acid. Still another object of the invention is to use a diffusion coating material which costs less than organosilicon compounds.

Yet another object of the invention is to apply the diffusion coating to a bulb by a slurry coating method using a water soluble organic binder which will loosely space the silica particles one from another to give effective diffusion, filament hiding, and good adherence of the particles to the glass bulb. A further object of the invention is to disperse the diffusion coating particles in a media which is nontoxic and explosion proof.

The objects of the invention are accomplished by coating a lamp bulb with a slurry containing silica particles, having an average particle size, measured by a scanning electron microscope, between 0.5 and 1.2 microns, suspended in a polyacrylic acid binder in ammoniacal water solution. This mixture of silica particles, polyacrylic acid, ammonium hydroxide and water is relatively nontoxic, nonflammable, and because it is aqueous, it does not create a silica inhalation hazard. Furthermore, the silica particles are obtained through the inexpensive method of ball milling natural silica.

Coating thickness may be varied by changing the urscosity and the solid content of the slurry. Variations of viscosity and solid content will change the silica thickness which, in turn, will give three different types of diffusion coatings: inside frost, Q-coat and enamel. Coating densities which correspond to the three types of bulbs are approximately 0.070 20.041, 0.315 $0.070, and 0.462 $0.072 mg/cm It has been found that the particles used may be either amorphous or crystalline silica. In the preferred embodiment, microcrystalline alpha quartz from the Pfizer Company is ball milled down to a particle size of between L9 to 2.1 microns as measured by a Coulter Counter. The mill charge is made of silica, water, polyacrylic acid and a basic hydroxide such as ammonium hydroxide. More water and ammonium hydroxide are added after milling until the desired viscosity and a pH of about 10 is obtained.

This slurry solution is then squirted into the inside of a clear, unetched glass bulb. After the bulb drains, there is a preliminary hot air drying which sets the coating on the bulb. The coating isthen lehred to remove the polyacrylic acid, ammonium hydroxide and water, which are presumably discharged from the inside of the bulb as CO N and H vapor.

It is believed that because the silica particles are sur rounded by polyacrylic acid molecules and because the polymer molecules are dissolved and ionized in water, an inorganic solvent, as opposed to prior art organic solutions, the silica particles are loosely spaced from one another, but tightly adhered to the bulb wall. It is also believed that loosely spaced particles make good diffusion coatings while still maintaining a satisfactory level transmission and good filament hiding power.

Other objects and advantages will become apparent from the following specification and drawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a fragmented perspective view of an incandescent lamp of the invention;

FIG. 2 is a fragmented perspective view of another type of lamp of the invention;

FIG. 3 is a front elevation in section of a part of the apparatus used in coating bulbs of the invention;

FIG. 4 is a photograph, magnified 2000 times, of a coating used to give the diffusion properties of an inside frosted lamp;

FIG. 5 is a photograph, magnified 2000 times, of a coating which has diffusion properties similar to a soft white or Q-coat lamp bulb;

FIG. 6 is a photograph, also magnified 2000 times, of a coating which can be used as a substitute for enameltype lamps;

FIG. 7 is a graph showing light absorption of a soft white lamp of the prior art;

FIG. 8 is a similar graph showing light absorption of the lamps of the invention; and

FIG. 9 is a Coulter Counter particle distribution of silica particles in the coating slurry.

DESCRIPTION OF THE PREFERRED EMBODIMENTS Referring to FIG. 1 of the drawings, an incandescent lamp 20 with a silica coating 21 is therein illustrated. Lamp 20 is comprised of bulb or envelope 22, mount 23 and base 24. The mount 23 has a tungsten filament 25 connected to lead wires 26 and 27 which are sealed in stem press 28. Envelope 22 and stem press 28 are hermetically sealed, and exhausted through tube 29. After exhausting, the envelope may be filled with an inert gas or gas mixture such as nitrogen and argon (95% argon, 5% nitrogen).

One of the requisites of light-diffusing coating 21 is that it must adhere to the bulb wall throughout the various lamp processing operations. The coating must also be an effective, efficient light diffuser and transmitter. Two basic measurements of diffusion coatings used to evaluate their effectiveness are light absorption (arbitrary units) of a coated lamp versus a clear lamp or standard light source and attenuated beam reading by a photocell versus horizontal scan along the bulbs major diameter.

Through the use of silica particles of a certain size;

true for both crystalline and amorphous silica. It should i be pointed out that the particle size range of 0.5 to 1.2 microns is the size of the particles on the envelope wall,

also referred to as final particle size. The other particle.

size range referred to in the application is the size of particles in the slurry prior to application to the envelope wall. The preferred range of the slurry particles is 1.9 to 2.1 microns, as measured by a Coulter Counter. An overall range of slurry particle size is 1.3 to 4.4 microns as measured by the Coulter Counter. This corresponds to the final particle size range previously recited 0.5 to 1.2 microns.

The coating of the invention gives good diffusion while absorbing approximately less than 6% of the light as compared to a clear bulb. Prior art slurry coatings having final particle size of 2.0 microns gave between 8 and 15% absorption. These prior art slurries use an organic solvent which has a tendency to agglomerate silica particles to a greater degree than the slurry coating of the invention.

In the coating of the invention, the silica particles are well dispersed in a strongly alkaline water medium. With the addition of ammonium hydroxide, the polyacrylic acid molecules become ionized and therefore enhance dispersion and limit agglomeration. Although polyacrylic acid is a preferred binder, the following may also be used: carboxy-methylcellulose and polyethylene oxides. such as Polyox 750 and Polyox 3000 produced by the Union Carbide Company. Other coating materials such as calcium pyrophosphate, soda-lime glass, aluminum silicate and alumina have been applied to glass lamp bulbs as substitutes for the preferred Williams Super White silica.

Coatings having a final particle size below 0.5 microns selectively scatter blue and purple light energy to a point where the filament has an undesirable red color. On the other hand, coatings having final particle sizes greater than 1.2 microns lack continuity and have a blotchy appearance. As coating final particle size increases near 1.2 microns, a slightly aesthetically inferior lamp is obtained although its diffusion and illumi-.

nating properties are good up to and including 1.2microns. A lamp with this type of coating could be used in locations where the aesthetics of the lamp are not important.

FIG. 9 is a Coulter Counter log-log graph of particle 1 size versus percent by weight of silica particles used in a slurry of a preferred embodiment which has a slurry particle size of 1.9 to 2.1 microns. By way of explanation, the Coulter Counter senses the volume of a particle and then converts the volume of the particle into a hypothetical sphere of the same volume having a diameter equal to the particle diameter. These Coulter Counter measurements are referred to as slurry particle size or slurry particle diameter. Ascan be seen from the graph, by weight of the particles in the slurry are of a size greater than one micron, and less than 1% by weight of the particles are greater than 6 microns. Furthermore, the numerical preponderance by weight percent of the particles are greater than one micron and greater than 0.8 microns. Coulter Counter measurements, unlike scanning electron microscope measurements taken at lOl5,000X magnification, are volumetric measures of agglomerates of more than one particle. In other words, three final particles of 0.5 microns as measured by the SEM may be measured as one slurry particle-of 1.5 or less microns in diameter as measured by the Coulter Counter when the three particles are attached to each other.

The particular slurry illustrated in the graph was obtained by ball milling for 27 hours at 62 rpm the following ingredients: distilled or deionized water (3800 gm), a wetting agent such as Antarox BL-225, GAF Corporation (0.096O gm), Acrysol A5, Rohm and Haas (88.5 gm), Williams Super White silica, Pfizer Chemical (680 gm), and a quantity of a basic hydroxide such as ammonium hydroxide sufficient to make the pH equal to 0.5. To this slurry is added 240 gm distilled water and an additional amount of ammonium hydroxide sufficient to maintain the above pH value. More or less water can be added until the reflectance, at the major diameter of an A-line bulb, is 34:1. This corresponds to a liquid viscosity of 55 i8 cps at 25C, as measured by a Brookfield Viscosimeter using a No. l spindle at 100 rpm. The percent solids in the above mixture is approximately 12.3 i0.4%. This mixture would give a diffusion coating similar to an enamelled lamp, and the coating density would be approximately 0.462 $0.072 mg/cm A coating which can be used as a substitute for inside frost is prepared in a similar manner. This coating has a reflectance of 10 $2, a viscosity of 36 i8 cps at 25C and 3.0 i0.l% solids. With the preceding suspension, a coating density of 0.070 i0.04l mg/cm should result. To obtain a coating which can be used as a Q-coat, the solution parameters should be reflectance of 25 i2, viscosity of 36 i8 cps at 25C and between 6.3 to 6.7 percent solids in the solution. This generally will result in a coating density of 0.315 10.070 mg/cm The diffusion coating of the invention may be used for lamps other than the exposed filament type shown in FIG. 1. It can be used, for instance, in lamp 30 shown in FIG. 2. Lamp 30, like lamp of FIG. 1, has a silica coating 31 of the invention on the inside wall of bulb 32. Instead of a filament, however, lamp 30 contains an inner lamp 33 which may be a tungsten-halogen lamp, as illustrated, or a discharge lamp. Actually, the diffusion coating of the invention may be used on any lighttransmitting substrate which is associated with a light source of some kind.

As can be seen from graphs 7 and 8, the diffusion coating of the invention differs from the standard Q- coat bulb with silica smoke deposited from the burning ethyl orthosilicate. All test readings used in compiling these curves were taken from l00-watt CC8 filament incandescent lamps. Curve A of FIG. 7 indicates the amount of red light transmitted by a standard Q-coated lamp, the readings being taken by a red sensitive photocell. Curve B which is a compilation of readings from a blue sensitive photocell shows a smaller amount of blue transmission. This disparity between red and blue transmissions will make the filament and a portion of the light it gives off appear red.

By way of contrast, curves A and B of FIG. 8 which represent the photocell response of the lamp of the invention show an almost equal amount of red and blue transmissions. The data for curves A and B of FIG. 8, like the data for the curves of FIG. 7, represent the respective responses of blue and red sensitive photocells.

. Since curves A and B of FIG. 8 are almost identical,

this means the light transmitted by the lamp will be white and not biased by an excess of red or blue. In many lighting applications, this balanced white light is a very desirable property.

Another advantage of using silica particles of a size within the range of the invention is the fact that they will absorb less moisture. This is desirable because bulbs can be manufactured and stored, whereas, bulbs of the prior art had to be made into lamps shortly after they were coated.

One of the objects of the invention is to find a process which is sufficiently versatile to be able to make the three basic types of diffusion coatings, namely: inside frost, soft white or Q-coat, and enamel. Inside frost is obtained by etching the inside surface of a clear bulb with hydrofluoric acid. Soft white or Q-coat is a twostep process. First, the bulbs are etched as in inside frosting, and then the etched bulbs receive a deposit of silica smoke from the combustion of an organosilicon compound. This coating requires the use of two steps and two separate pieces of equipment. Outside enamel is painting the exterior of the bulb, a process which requires yet another and different piece of equipment. Accordingly, it would be quite desirable from an economic standpoint to be able to make the three types of coatings using the same facilities and machinery.

This objective is accomplished by varying the density (mg/cm of the coating of the invention by changing the slurry reflectance, viscosity and percent solid as previously described. Referring now to FIGS. 4, 5 and 6, the photographs shown therein represent varying desities of silica on glass bulbs corresponding to the three types of diffusion coatings. In FIG. 4, the silica particles are loosely packed with respect to each other and their density on the bulb wall is comparatively small. The approximate density of these particles is 0.070 $0.041 mg/cm This coating would serve as a substitute for inside frost, acid etched bulbs. Inside frost bulbs have less diffusion than other coatings which is why the particle density shown in FIG. 4 is smaller than that of FIGS. 5 and 6.

A distinguishing characteristic of the coating of the invention is the irregular shape of some of the silica particles. In other words, the particles are not totally spherical. This characteristic of not all particles being spherical can be observed in all three figures.

FIG. 5 shows a coating with particle density greater than that shown in FIG. 4 but less than that shown in FIG. 6. This density is approximately 0.315 $0.070 gm/cm and corresponds to the diffusion of a soft white or Q-coat lamp, acid etched plus silica smoke. It can be observed in FIG. 5 that there are agglomerates or large particles, and small particles, presumably single particles. It is believed that this combination of agglomerate and small particles performs the same function as etched glass and small silica particles do in Q-coat.

In FIG. 6, there is shown a silica coating with a density of 0.462 10.072 gm/cm which corresponds to an outside enamel coating. As can be observed, this coating has the greatest density of all three specimens. All photographs in the figures are magnified 2000 times.

Part of the apparatus and method used in depositing the coating is illustrated in FIG. 3. The coating apparatus 34 is comprised of a slurry holding tank 35 which has an air inlet 36, slurry inlet 37 and applicator nozzle 38. Holding tank 35 is sealed from the atmosphere by plate 39, the top part of which forms spillway 40. Bulb 41 can be held by numerous types of holding means such as the one shown at 42.

Once bulb 41 is positioned in holding means 42 over applicator nozzle 38, air pressure is applied to the slurry through opening 36. The pressure increases in the space above the slurry forces some of the slurry up applicator nozzle 38 onto the inside of the bulb wall. Any excess slurry falls into spillway 40 and can be recylced and reused. Slurry replenishment comes through the nozzle and opening shown at 37.

After the bulb 42 has been coated, an air jet is inserted to give the coating a quick dry. Following this, the bulbs are lehred in an oven according to the following oven temperature schedule: heat up from room temperature to 780C, bake at 780C for seconds, bake at 680C for seconds, cool to upper strain point of approximately 525C in 5 seconds, cool to lower strain point of approximately 475C in 60 seconds and cool to room temperature in approximately 60 seconds.

What I claim as new and desire to secure by Letters Patent of the United States is:

l. A glass electric lamp bulb having on the inside surface thereof a light-diffusing coating of silica particles adhering to said surface, randomly spaced from one another, and having a diameter within the range from about 0.5 to 1.2 microns, said coating characterized by a density of approximately 0.030 to 0.535 milligrams per square centimeter.

2. The glass electric lamp bulb claimed in claim 1 wherein said silica particles are crystalline silica.

3. The glass electric lamp bulb claimed in claim 1 wherein said silica particles are amorphous silica.

4. The glass electric lamp bulb claimed in claim 1 wherein the numerical preponderance by weight of said silica particles have a diameter larger than the longest wavelength of visible light.

5. The glass electric lamp bulb claimed in claim 1 wherein the luminous efficiency in lumens per watt of an ordinary incandescent lamp comprising such a coated bulb is of the order of 1.5 to 6% less than that of the same lamp with a clear bulb.

6. The glass electric lamp bulb claimed in claim 1 wherein said particles of silica have a diameter within the range from 0.6 to 0.7 microns.

7. The glass electric lamp bulb claimed in claim 1 wherein said particles of silica have a diameter within the range from 0.5 to 1.0 microns.

8. The glass electric lamp bulb claimed in claim 1.

wherein a portion of said particles of silica are characterized as being nonspherical.

9. The glasselectric lamp bulb as claimed in claim 1 wherein said coating density is approximately 0.070 10.041 mg of silica per cm 10. The glass electric lamp bulb as claimed in claim 1 wherein said coating density is approximately 0.315 10.070 mg of silica per cm 11. The glass electric lamp bulb as claimed in claim 1 wherein said coating density is approximately 0.462 120.072 mg of silica per cm 12.. An incandescent lamp comprising a glass bulb having on the inside surface. thereof a light-diffusing coating of silica particles adhering to said surface, randomly spaced from one another, and having a diameter of about 0.5 to 1.2 microns, said coating characterized by a density of approximately 0.030 to 0.535 milligrams per square centimeter, and a filament mounted inside said bulb, said coating serving a dual function of a light-diffusing means and as a means for preventing the dislodgment of any impurities from the inside surface of the glass bulb which would tend to lower the breakdown voltage across said filament.

13. The lamp claimed in claim 12 wherein said bulb contains a fill gas and said tightly packed coating of sil-. ica prevents'the dislodgment of any impurities from the bulb surface which would combine with the fill gas. 

1. A glass electric lamp bulb having on the inside surface thereof a light-diffusing coating of silica particles adhering to said surface, randomly spaced from one another, and having a diameter within the range from about 0.5 to 1.2 microns, said coating characterized by a density of approximately 0.030 to 0.535 milligrams per square centimeter.
 2. The glass electric lamp bulb claimed in claim 1 wherein said silica particles are crystalline silica.
 3. The glass electric lamp bulb claimed in claim 1 wherein said silica particles are amorphous silica.
 4. The glass electric lamp bulb claimed in claim 1 wherein the numerical preponderance by weight of said silica particles have a diameter larger than the longest wavelength of visible light.
 5. The glass electric lamp bulb claimed in claim 1 wherein the luminous efficiency in lumens per watt of an ordinary incandescent lamp comprising such a coated bulb is of the order of 1.5 to 6% less than that of the same lamp with a clear bulb.
 6. The glass electric lamp bulb claimed in claim 1 wherein said particles of silica have a diameter within the range from 0.6 to 0.7 microns.
 7. The glass electric lamp bulb claimed in claim 1 wherein said particles of silica have a diameter within the range from 0.5 to 1.0 microns.
 8. The glass electric lamp bulb claimed in claim 1 wherein a portion of said particles of silica are characterized as being nonspherical.
 9. The glass electric lamp bulb as claimed in claim 1 wherein said coating density is approximately 0.070 + or - 0.041 mg of silica per cm2.
 10. The glass electric lamp bulb as claimed in claim 1 wherein said coating density is approximately 0.315 + or - 0.070 mg of silica per cm2.
 11. The glass electric lamp bulb as claimed in claim 1 wherein said coating density is approximately 0.462 + or - 0.072 mg of silica per cm2.
 12. An incandescent lamp comprising a glass bulb having on the inside surface thereof a light-diffusing coating of silica particles adhering to said surface, randomly spaced from one another, and having a diameter of about 0.5 to 1.2 microns, said coating characterized by a density of approximately 0.030 to 0.535 milligrams per square centimeter, and a filament mounted inside said bulb, said coating serving a dual function of a light-diffusing means and as a means for preventing the dislodgment of any impurities from the inside surface of the glass bulb which would tend to lower the breakdown voltage across said filament.
 13. The lamp claimed in claim 12 wherein said bulb contains a fill gas and said tightly packed coating of silica prevents the dislodgment of any impurities from the bulb surface which would combine with the fill gas. 